1. Field of the Invention
This invention relates to pneumatic tires, and more particularly to a pneumatic radial tire for passenger car having a directional tread pattern. The tread pattern is defined by a circumferential center groove continuously extending in parallel to a circumferential direction of the tire or substantially in parallel thereto, a plurality of directional slant grooves disposed at intervals in the circumferential direction of the tire and extending obliquely with respect to the circumferential direction of the tire, and a plurality of blocks formed at intervals in both circumferential and axial directions of the tire and having an acute corner portion. The rotating direction of the tire in the mounting onto a vehicle is specified to firstly contact a portion of the directional slant groove extending toward a side of an equatorial plane of the tire with ground and lately contact a portion thereof far away from the equatorial plane with ground.
Throughout the specification, the term "directional slant groove" means a groove extending obliquely in the circumferential direction and forming so-called directional tread pattern wherein the rotating direction (forward running direction) of the tire in the mounting onto a vehicle is specified so as to firstly contact a portion of such a groove extending toward a side of an equatorial plane of the tire with ground and lately contact a portion thereof far away from the equatorial plane with ground.
2. Description of Related Art
Recently, the pneumatic radial tire for passenger car is demanded to have a high drainage performance in the running on wet road surface as compared with the conventionally known tires.
In order to enhance the drainage performance in the running on wet road surface, there is proposed and used a pneumatic tire having a directional tread pattern defined by a plurality of circumferential grooves disposed at intervals in an axial direction of the tire and extending in parallel to or substantially in parallel to the circumferential direction of the tire, a plurality of directional slant grooves disposed at intervals in the circumferential direction of the tire and extending obliquely in the circumferential direction of the tire to open to the circumferential grooves, and a plurality of blocks formed at intervals in both circumferential and axial directions of the tire, and specifying the rotating direction of the tire in the mounting onto a vehicle is specified to firstly contact a portion of such a groove extending toward a side of an equatorial plane of the tire with ground and lately contact a portion thereof far away from the equatorial plane with ground. In this case, the circumferential grooves are used for drainage in the front and rear directions and the slant grooves are used for drainage in lateral direction.
In such a tire, however, the circumferential grooves and the slant grooves extending in different directions with respect to the circumferential direction of the tire intersect with each other, so that water flows collide with each other at such intersect portions to cause disorder of the water flow or create bubbles and finally a high drainage performance is not obtained.
Also, if it is intended to enhance the drainage performance in the running on wet road surface in the conventional tire, it tends to cause a problem of degrading the steering stability in the running on dry road surface.